Vegetable oil alkyl esters intended to be used as biofuel are produced from vegetable oils obtained for example from rapeseed, sunflower, soybean or even palm. III-suited for directly feeding modern diesel engines of private cars, vegetable oils essentially consisting of triglycerides have to be converted by means of a transesterification reaction with an alcohol, methanol or ethanol for example, introduced in excess to produce vegetable oil methyl esters and glycerin.
This reaction is schematized as follows:1 triglyceride+3 alcohols→3 fatty matter esters+glycerin
In cases where the transesterification reaction is coupled with an esterification reaction, the reaction schemes are as follows:Fatty acid+alcohol→fatty acid esters+waterFatty acid+glycerin→glyceride+water
Alkyl ester production methods have already been developed. Some use the conventional routes of homogeneous catalysis with soluble catalysts such as soda or sodium methylate, by reacting a neutral oil and an alcohol. Examples thereof are the methods described in patent EP-0,523,767, with continuous use of a basic homogeneous catalyst, or in patent FR-B1-2,890,961 filed by the applicant, which describes an ethyl ester production method. However, this type of method involves several drawbacks. It is in fact necessary to implement a complex and laborious chain of treatments in order to reach the specifications sought for the glycerin and the ester produced.
Heterogeneous catalysis methods have been described. Patent FR-B1-2,752,242 describes a method of producing a fatty acid ester and glycerin of high purity from a vegetable oil and an aliphatic monoalcohol in the presence of a solid catalyst based on zinc aluminate. Patents FR-B-2,855,517, FR-B-2,855,518 and FR-B-2,855,519 describe alkyl ester production methods involving catalysts based on mixed oxides comprising titanium, zirconium, antimony and/or aluminium. Patents FR-B-2,869,612 and FR-B-2,869,613 describe the use of catalysts based on zinc, bismuth, titanium and/or aluminium oxides. More recently, patent FR-B-2,914,927 describes the use of a heterogeneous catalyst based on phosphate or an organophosphorus compound of group 4. Patent application WO-2007/043,062 that describes a transesterification method using acid catalysts based on zinc ferrocyanide can also be mentioned.
Heterogeneous methods allow to obtain high alkyl ester yields, as well as a glycerin of high purity. The latter point has a very significant impact on the economy of the method since the price of glycerin essentially depends on its purity level.
Vegetable oils of terrestrial or aquatic origin, or animal fats used as raw material in these methods are mainly made up of triglycerides, in a proportion of 80-98 wt. %. The minor constituents, present in a proportion of 2 to 20 wt. %, are free fatty acids, mono- and diglycerides, glyceride oxide compounds resulting from the degradation of oil, waxes (natural hydrocarbons present in oils), proteins containing sulfur and/or nitrogen, phospholipids, tocopherols, sterols, natural colorants, as well as more or less volatile odorant compounds. Crude oils can also contain as minor compounds species containing heteroelements such as phosphorus, magnesium, calcium, iron or zinc, in proportions up to 2500 ppm, mainly in form of phospholipids and/or sterols in the case of phosphorus, magnesium and calcium, or present in pigments in the particular case of magnesium and in form of sterols in the case of iron and/or zinc (Oils and fats manual: A comprehensive treatise, Volume 1, page 90, Karleskind A. et al.).
In order to eliminate these minor compounds, the pressed or extracted crude oil is conventionally refined so it can be used. For use as biofuel, the conventional oil refining stages are as follows:                degumming consists in removing the phospholipids or mucilages that precipitate in the presence of acidulated water,        neutralizing the degummed oil in the presence of a soda solution allows to neutralize at least part of the free fatty acids present in the oil. The soap stocks formed during this stage carry along part of the impurities contained in the oil,        water wash to eliminate sodium salt traces,        vacuum drying.        
The oil obtained after this refining process is referred to as semi-refined oil. Other operations such as decolorization and deodorization are actually necessary to obtain food grade oil or “refined oil”.
However, after refining, the oils referred to as semi-refined can still contain up to 20 ppm phosphorus, calcium, magnesium, in form of phospholipids, and iron or zinc in form of sterols (Oils and fats manual: A comprehensive treatise, Volume 1, page 90, Karleskind A. et al.).
Under the temperature conditions of the heterogeneous transesterification method, these species are converted to mixed calcium and magnesium phosphate of CaxMgy(PO4)z type, insoluble in the reaction medium. These solid species then settle in the catalytic bed and involve delicate operation of the reactor, with a pressure drop increase in the reactor and catalyst deactivation through pore clogging. These species can also leave the reactor and be carried in the glycerin treatment circuit. The quality of the glycerin is then degraded by the presence of these solid species. In the particular case of a process operating in two stages with partial recycle of elements of the glycerin separation chain (glycerin, methanol and unconverted species) as described in patent FR-B-2,838,433, these impurities circulate then in different places in the process, even upstream from the reactor, and cause operating clogging problems. In order to make this process operable, the allowable feeds have been limited to the semi-refined or DND (Degummed, Neutralized and Dried) feeds.
On the other hand, in addition to phospholipids, these oils naturally contain glucosylated sterols (up to 500 ppm depending on origins) that can come in two different forms: acylated and non-acylated glucosylated sterols, which cause clogging problems in the heterogeneous transesterification reactor and in the final esters as regards their resistance to cold, in particular non-acylated glucosylated sterols (Robert A. et al., J. Am. Oil Chem. Soc. (2008) 85: 761-770). Now conventional oil refining does not allow them to be eliminated.
Patent application WO-2007/098,928 concerns a method for physical refining of feeds based on triglycerides used in biodiesel preparation methods. After a degumming stage, the triglycerides are contacted with adsorbent particles, at atmospheric pressure, at temperatures ranging between 60° C. and 90° C., in order to decrease their phosphorus content. This adsorption stage is followed by a filtration stage, then the product obtained is subjected to a stripping stage intended to reduce the free fatty acid content.
The method provided comprises a succession of several stages, which involves relatively high operating costs. On the other hand, the temperatures at which the oil refining stages are carried out do not exceed approximately 100° C. The processes involved are adsorption-on-solid phenomena, known to operate in an optimized manner at relatively low temperatures.
It seems necessary to be able to control and limit the proportion of clogging insoluble species in order to improve the operability of industrial plants, without introducing too large a number of stages in the processes so as to limit operating costs. The activity and the life of the catalyst, as well as the quality of the esters and of the glycerin co-produced, can thus be improved.
An objective of the present invention is to provide a vegetable or animal oil transesterification method comprising a crude or semi-refined oil pre-treatment stage allowing to eliminate the hetero-elements contained in the minor compounds such as phosphorus, magnesium, calcium, iron and/or zinc.
Consequently, this invention makes it possible to use a crude oil that has undergone no pre-treatment or a semi-refined oil as defined above as the feed in the method of transesterification of oils by an alcohol. Furthermore, pre-treatment allows concentration of the impurities and conversion thereof to a valorizable solid (fertilizer for example).